1. Field of the Invention
The present invention relates to a device for detecting vehicle speed of a vehicle, particularly an ordinary four-wheeled automobile, for the purpose of providing a base of a due slipless rotation speed of the wheels for use in operating the so-called antilock brake system.
2. Description of the Prior Art
The antilock brake system (ABS) is already well known as a brake system of a vehicle such as an automobile which automatically decreases a braking force applied thereby to a wheel or wheels of the vehicle when the braked wheel starts to slip so much as to lose its gripping of the road surface due to an excessive braking force applied thereto.
In order for the antilock brake system to operate at a high appropriateness, it is essential that the vehicle speed, i.e. the running speed of the vehicle body, is correctly measured or estimated to provide the base of a due slipless rotation speed for the respective wheels, so that the slipping or locking condition of each braked wheel is correctly judged from a difference between the due slipless rotation speed and an actual rotation speed of the wheel. Unless the running speed of the vehicle body is directly detected by an expensive high technology device such as a supersonic or laser speed detector, the measurement thereof will have to depend on the measurement of the rotation speed of one or more of the wheels. However, since the rotation speed of each wheel is liable to deviate from such a due slipless rotation speed because of the applications of driving or braking torques thereto and/or irregularities of the road surface, there have been made various contrivances to derive the vehicle speed as correctly as possible from the rotation speeds detected with respect to all or some of the wheels of the vehicle, such as to select the highest or lowest value of them or to calculate a simple or a weighted mean value of some or all of them.